1. Field of the Invention
The present invention relates to an optical pick-up device for recording and/or reproducing information for an information record medium having a guide track layer and plural record layers in depth direction.
2. Related art Statement
Such a kind of optical pick-up device has been described in for example U.S. Pat. No. 5,408,453. This optical pick-up device performs recording/reproducing of the information by using a guide track layer at the side near the objective lens, and using the guide light source and the scanning light-source for the information recording medium having the plural record layers at a side away from the objective lens, so that the guide beam from the guide light source is converted into the collimated light flux with the guide collimator lens, and then focused on the guide track layer of the information recording medium through the beam combining element, the galvanometer mirror, and the objective lens. Moreover, the scanning beam from the scanning light source is converted into the collimated light flux with the scanning collimator lens, and then combined with the guide beam by the above beam combining element by adjusting the settling degree and the emanation degree by the optical axis direction deflector and then focused on the record layer at the desired depth of the information record medium through the above galvanometer mirror and the objective lens.
Thus, in the information recording, the guide beam is focused to the guide track layer, and the focusing control and the tracking control are performed based on the return light thereof, and while controlling parallelism of the scanning beam incident on the objective lens with a shifter in the direction of optical axis, the information is recorded by focusing the scanning beam on to the record layer at the desired depth, and then in reproducing and the deletion of information recorded on the record layer at the desired depth, the scanning beam is made to focus on the record layer at the desired depth, and information is reproduced and erased while performing the focus control and the tracking control based on the return light.
By the way, in the case of recording/reproducing information by focusing the guide beam on the guide track layer of information recording medium by the above common objective lens, and by focusing the scanning beam on the record layer located in the desired depth which shifts in optical axis direction, if the collection position of the scanning beam, that is, the position of the record layer is changed and the medium thickness to the scanning beam is changes, the spherical aberration is caused in the scanning beam spot according to the change in the medium thickness, so that the spot becomes large, and thus recording/reproduction performance is deteriorated due to decrease of peak intensity, and stable recording/reproducing to each record layer can not be performed in the depth direction.
Then, in the above conventional optical pick-up device, the aberration correcting element having different thickness area is arranged respectively in the optical paths between the light source and the collimator lens and between the light detector and the collimator lens, these aberration correcting elements are driven independently to locate the given thickness area in the optical path in accordance with the depth position of the selected record layer, thereby correcting spherical aberration.
However, in the above conventional optical pick-up device, the independent aberration correcting element is required for the light source and the light detector, and the driving mechanism for driving these aberration correcting elements independently, is required, so that the number of components become increased and the constitution becomes complicated and large so that the cost-up will be invited.
Moreover, the scanning beam is made incident on the objective lens with collimated light flux and The scanning beam is made incident on the objective lens by adjusting the convergence degree and the divergence degree according to the depth position of the record layer, so that Intensity distribution of the scanning beam incident on the objective lens is changed according to the position of the record layer and the change in diameter of diffraction spot, the aberration deterioration, and the shading or the like are caused due to the scanning beam incident on the objective lens as the convergence light or the emanation light, therefore, the recording/reproducing performance becomes deteriorated.
To prevent deterioration in the recording/reproducing performance, the scanning beam is made incident on the objective lens in the parallel light flux, and the objective lens is moved in optical axis direction according to the depth position of the record layer, so that it is preferred that the guide beam is made incident on the guide track layer so as to focus the guide beam by adjusting its parallelism.
However, in this case, when the collecting position of the scanning beam is changed, the spherical aberration is caused in not only the scanning beam spot but also the guide beam spot formed on the guide track layer, of which the medium thickness is not changed, so that the focusing control and the tracking control become unstable.
For example, in the case of using an objective lens, of which the spherical aberration is corrected for wave length 650 nm, assuming that the wave length of the scanning beam is 650 nm and the wave length of the guide beam is 780 nm, the refractive index falls for wave length 780 nm, so that the lens power becomes weaken, and thus the spherical aberration is generated at an over side. Moreover, in order to make WD of the objective lens (working distance) constant and to focus the guide beam on the guide track layer, the elongation amount of optical path length (the length shown by the state which has no plastic substrate (protective layer 5) to guard plural record layers, that is, the air conversion length shown by the length converted into refractive index (1.0) of air.) due to the chromatic aberration must be amended by thickening the medium thickness, and the spherical aberration is caused on an over side by the increase of the thickness.
Therefore, when NA of the objective lens is assumed to be for example 0.52, the spherical aberration of about 0.02 xcexrms or more will be generated on the over side.
Moreover, when the guide beam is made incident on the objective lens as the converging light or the emanation light, by moving the guide collimator lens to the optical axis direction in accordance with the movement of the optical axis direction of the objective lens according to the depth position of the record layer, in order to focus the guide beam to the guide track layer, the amount of the spherical aberration, which causes on the guide beam spot, is changed according to it. Particularly, when the guide beam is made incident on the objective lens as converging light, the spherical aberration is further added to the over side for the offset of the spherical aberration of about 0.02 xcexrms or more at an over side, so that an increase in the yield of the spherical aberration becomes remarkable. Therefore, in case of performing the focus control and the tracking control based on the return light of the guide beam from the guide track layer, those controls become unstable.
In addition, in case of making the numerical aperture of the guide collimator lens to be xc2xd or less compared to the objective lens, for example, to be 0.2 usually, in order to generate the spherical aberration by which the spherical aberration (0.02 xcexrms) generated in the guide beam spot is counterbalanced, a very thick aberration correcting element of 3 mm in thickness should be used as an aberration correcting element inserted between the guide light source and the guide collimator lens, since the spherical aberration is proportioned to the fourth power of the numerical aperture of the guide collimator lens Moreover, when the refractive index of the aberration correcting element is assumed to be 1.5, the guide collimator lens should be shifted in the optical axis direction by 3 mmxc3x97(1{fraction (1/1.5)})=1 mm, together with interlocking to the insertion of the aberration correcting element in the optical path. Thus, not only the aberration correcting element with thick thickness is needed, but also the moving mechanism of guide collimator lens is needed, so that the constitution becomes complicated further more and large-scaled, and thus the cost-up will be invited.
Moreover, in case of moving the scanning beam spot in the depth direction in the medium, over xc2x150 xcexcm, the amount of the movement of the guide collimator lens becomes very much, so that the moving mechanism have a bigger burden and a vignetting of the guide beam is generated. Particularly, in the optical pick-up device of type performing the high-speed seeking by the movable optical system in the radius direction of the disc, and having a long optical path length of which the optical system is separated by a the fixed optical system, in which a semiconductor laser and a light detector or the like are arranged, and a movable optical system, in which an objective lens or the like are arranged, the vignetting of the guide beam becomes very much.
Therefore, the present invention has for its object to eliminate the above disadvantage of the conventional optical pick-up device. The present invention has for its object to provide an optical pick-up device capable of recording/reproducing information stably on respective record layers in the depth direction with easily, small, and cheaply.
There is described an optical pick-up device for optically recording and/or reproducing information on an information record medium having plural record layers and guide tracking layers in the depth direction comprising: a first light source for radiating first light beam, a second light source for radiating second light beam which is different from the first light beam, plural objective lenses selectively arranged in a common optical path for the first light beam and the second light beam, for focusing first light beam and second light beam with different spot spacing, objective lens selection means for selecting objective lens arranged in a common optical path of plural objective lenses according to depth position of desired record layer to be recorded and/or reproduced of the information recording medium, thereby focusing the first light beam on the guide track layer through the objective lens arranged in the common optical path, and focusing the second light beam on the desired record layer.
According to the invention of the claim 1, the objective lens corresponding to the objective lens selection means is selected according to the depth position of the desired record layer to be recorded and/or reproduced, the first light beam is focused on the guide track layer through the selected objective lens and the second light beam is focused on the desired record layer, so that deterioration in recording/reproducing performance to respective record layers in the depth direction can be effectively prevented by an easy, downsized, cheap constitution, and stable servo signal can be obtained from the return light of the first light beam, and thus recording/reproducing to respective record layers becomes possible with stable.
In the optical pick-up device according to claim 1, the hologram element capable of arbitrarily setting the spot spacing of the first light beam and the second light beam due to the objective lens in the information record medium is provided, in accordance with at least one of plural objective lenses, thereby arranging the hologram element in the common optical path together with the objective lens.
According to the invention of claim 2, freely setting the spot spacing of the first beam and the second beam due to the objective lens can be set in correspondence with the hologram element, and the aberration of respective spots can be corrected.
In the optical pick-up device according to claim 2, the hologram element is formed to the corresponding objective lens.
According to the invention of claim 3, the hologram element is formed in the objective lens in a uniform shape, so that the cost can keep cheap compared with the case that these element and lens are formed separately, and the number of parts can be decreased and assembly can be facilitated.
In the optical pick-up device according to claims 1, 2 or 3, an objective lens driving means for controlling a relative position of the objective lens to the information record medium is provided by at least displacing the objective lens arranged in the common optical path in the given direction, during at the recording and/or reproducing time of information for the information record medium, thereby constituting the objective lens selection means together with including at least a part of the objective lens driving means.
According to the invention of claim 4, as for the objective lens arranged in the common optical path, a relative position to the information record medium is controlled by the objective lens driving means, so that while making the first light beam follow to the desired guide track of the guide track layer with the unfocussed condition, the second light beam to the desired record layer by the unfocussed condition can be unfocussed, and thus the recording/reproducing of more accurate information to respective record layers can be performed. Moreover, the objective lens selection means is constituted by including at least a part of the objective lens driving means, so that the objective lens selection means can be downsized with inexpensive and thus the number of parts can be decreased and assembly can be facilitated.